(1) Field of the Invention
This invention relates to a heating cable with a specific heating capacity for use in devices, applicances, structural parts, public squares, streets, etc.
(2) Description of the Prior Art
Heating cables are known which are single core or multi-core designs, and which generate heat energy by using electrical resistance wires. In addition, heating cables are also known in which at least two electrical conductors running essentially parallel are separated from one another by a resistance material, and in which this separator material exhibits fixed, specific electrical resistance values. Moreover, depending on where these heating cables are employed, said separator materials may be chosen so as to have either a positive or negative coefficient of resistance.
In the case of these known heating cables, cables with various resistance values must be available for different applications. Thus, the cables can be adapted first by means of selecting the length of the heating cable and secondly by means of using materials having different resistance values for the electrical wires or for the separator material. To adapt the cable in this manner, a compromise must always be struck since for economic reasons only a limited number of heating cables with different resistance values can be manufactured. Besides these economic considerations, heating cables which have various resistance values are also subject to engineering restraints.
In order to achieve optimal adaptation to different requirements where heating cables are used, it would be desirable to use heating cables with a specific heating capacity, in other words, with a specific wattage per unit of length.
A cable of this type with such a specific heating capacity is known. In this known cable type two conductors running parallel and insulated from one another are jointly wrapped with electrically conducting strips having the requisite resistivity so that these resistance strips make alternate contact with the parallel conductors. By choosing the intervals between these points where the resistance strips make contact with the electrical conductor or conductors, it is thus possible to maintain a specific heating capacity. Since such a resistance strip makes contact with both parallel conductors, i.e., with both the phase wire as well as with the neutral wire, in the case, for example, of 220 volt alternating current, said heating cable must be surrounded by a relatively rigid and thick covering of electrical insulating material. Of necessity this results in very poor outward heat conduction, i.e. it leads to high heat buildup inside the cable structure and consequently to changes in resistance and to unstable conditions. Furthermore, such heating cables are very hard to bend because of the engineering design features required, and this in turn leads to considerable difficulties when the cables are laid. On the basis of its construction and the engineering design features required, this known heating cable not only has a very high materials cost but above all is also very expensive to manufacture.